The present invention relates to a developing unit for an electro-photographic apparatus, such as a copier or a printer. More specifically, the present invention relates to a toner concentration sensor for the developing unit.
An electro-photographic apparatus generally includes a latent image forming drum coated with photosensitive material, a developing unit for developing the latent image into a visible image, and means for fixing the developed image onto paper.
The developing unit generally includes: a casing for containing developer, which is a mixture of a carrier and toner; a paddle roller for paddling the developer; a developing roller for carrying the developer to the latent image forming drum, and a toner concentration sensor for sensing and controlling the toner concentration of the developer and maintaining adequate developing quality.
In a conventional electro-photographic apparatus, the sensor is provided on a blade which regulates the thickness of the layer of developer disposed on the developing roller. One of the reasons the sensor is provided on the blade is that the sensor should sense the developer actually being supplied to the developing roller, i.e., as the developer is paddled up by the paddle roller and poured onto the developing roller. The developer scraped off by the blade must be of the same concentration as the developer actually used for the development of the latent image. Therefore, it is logical that measurement at the blade should accurately reflect the developing quality.
However, it has been found that the output signal of the sensor provided at the blade is insufficient for accurate control of the toner concentration. More particularly, the output of the sensor fluctuates according to fluctuation of flow of the developer at the blade.